Only show colour and NOT transparency?
There are generally four steps to make sure the viewport alpha display:


Set Transparency Alpha value to 0.000 in Material setting;
Toggle Alpha influence in Texture setting;
Switch to GLSL shading mode in 3D View Property panel;
Use Texture as viewport shading type. 




Besides, make sure Using Alpha is toggled in Texture panel, although it's set as default. And you may want to use UV as the coordinate type of texture mapping for most of the case.



However, it is interesting that you can even preview it without assigning any material, even in the default Multitexture shading mode (just like when you drag and drop an local image onto a object in the scene). And of course, it cannot be rendered, since no material assigned.